Cellulosic fibres, like wood and plant fibres, have the potential for use as load-bearing constituents in composite materials due to their attractive properties such as high stiffness-to-weight ratio that makes cellulosic fibre composites ideal for many structural applications. There is thus a growing interest among composite manufacturers for such low-cost and low-weight cellulosic fibre composites. In addition, wood and plant fibre based composites with thermoplastic polymeric matrices are recyclable, and they are cost attractive alternatives to oil based fibre reinforced polymer composites that currently have the largest market share for composite applications. However, the most critical limitation in the use of cellulosic fibre composites for structural applications is the lack of well described fibre properties, in particular, the tensile strength. This is due to variations in fibre morphology, fibre processing conditions, and applied test methods. Other limitations such as dimensional instability and low fibre-matrix adhesion have already been intensively investigated, and solutions have been found for many commercial applications. Therefore, a better understanding of the mechanical performance of these fibres, and with a focus on increasing their strength will make it possible for them to reach their full potential as reinforcement in composites. The present PhD study deals with several important subjects related to the use of flax fibres in composites. The emphasis is on the relationship between the complex microstructure and the tensile properties of flax fibres and their composites, based on textile flax yarn and a thermoplastic polymeric matrix. Single flax fibres were isolated from flax fibre bundles which have been processed in two differentnsteps of natural treatments (retting) and mechanical treatments (scutching and hackling). Microscopic observations of the defects formed in the fibres and their fracture surfaces after tensile testing show that large fracture areas are formed in a complex way due to defects in the fibre cell wall, and due to anisotropy of the internal cell wall structures. This is in contrast to the crack growth in brittle ceramic and glass fibres. Moreover, two typical stress-strain curves (linear and non-linear) measured for the flax fibres were found to be correlated with the amount of defected region in the fibres. The defects are induced in larger numbers and larger sizes during processing of the fibres, and this is found to be correlated with a decrease in tensile strength of the fibres. It is found that processing reduces the tensile strength from average values of 1450 MPa for naturally processed single fibres to 810 MPa for mechanically processed single fibres. The large variation in tensile properties of flax fibres leads to an examination of the effect of defects and applied test methods. The fibres show a large coefficient of variation (CV) in the range 20-60% in general for all measured tensile properties. One reason for these relative large variations can be attributed to the assumption of a circular cross sectional area of the fibres. On average, these results in a 39% lower tensile strength than when the true fibre cross sectional area is used, and moreover, the variable aspect ratio of the cross section of fibres significantly affects the variation of the results. Also, the large variation in properties is likely to be attributed to the distribution of defects along the fibres since the large defects lead to low mechanical properties, whereas smaller defects result in less reduced mechanical properties. On the level of composites, the effect of consolidation pressure on the tensile properties of flax fibre composites was investigated. A porosity corrected rule of mixtures model, and a volumetric composition model for composites were used to model the experimental data. Flax fibre yarns and thermoplastic low-melting temperature polyethylene terephthalate (LPET) filaments were aligned in assemblies of different fibre weight fractions in the range 0.24 to 0.83 to manufacture unidirectional composites using two different consolidation pressures of 1.67 and 4.10 MPa. The maximum attainable fibre volume fraction is found to be 47% for the low pressure composites, whereas it is found to be 60% for the high pressure composites. The stiffness of the flax fibre/LPET composites is measured to be in the range 16 to 33 GPa depending on the volumetric composition of the composites. The high pressure composites are found to have superior tensile properties in comparison with the low pressure composites. The tensile strength (mean ± std. dev.) of the low pressure composites was found to be 183±7 MPa while that of the high pressure composites was found to be 209±6 MPa at a fibre volume fraction of 22%. The effect of fibre correlated porosity and structural porosity in the composites is found to be highly important for the volumetric composition and tensile behaviour of the composites. The total porosity is measured in the range 2.4 to 32%, and it is found to be increased dramatically when the fibre weight fraction is increased above a transition value, as predicted by the volumetric composition model. This leads furthermore to a scatter in the experimental data of stiffness at high fibre weight fractions. The qualitative analysis of the composite cross sections by microscopy also shows that the low and high pressure composites have a similar microstructure at low fibre weight fractions. However, when the fibre content is increased, a difference in porosity content can be observed from the composite cross sections. The nominal tensile strength of the unidirectional flax fibre/LPET composites is measured in the range 180 to 340 MPa. However, in many cases, the tensile strength determined of unidirectional composites is not valid due to the fact that failure does not occur in the gauge section. It is actually common that unidirectional composites fail close to the grips, and they then split along the specimen in the tensile direction. Traditionally, the problem has been approached by the use of local reinforcement of the specimen in the gripping areas, the so-called tabs, but the problem has not been efficiently solved in practice. A key problem is that the stress state at the end of the tab can be singular, leading to premature failure of the tensile specimen. In the present study, the dependence of the order of the stress singularity at the vertex of dissimilar isotropic and orthotropic materials is investigated in terms of the elastic mismatches between the specimen and the tab materials, and the tab angle. Finite element modelling is performed to analyse the situation of a stress singularity. The results are aimed at creating a better specimen/tab design to accomplish failure in the gauge section of the tensile specimens, and thereby determine the true tensile strength of the materials. It is found that the stress singularity in the tab wedge is reduced with a decreased tab angle and with a decreased stiffness of the tab material. A simple criterion is proposed for the assessment of the severity of the stress singularity. In practice, gauge section failures should be achievable by selecting a test specimen design based on combinations of a stiff material in the tab section combined with a soft material (eg. epoxy adhesive) at the wedge end of the tab, forming a wedge. The wedge tip should have a small wedge angle in the range 5° and 10° depending on the stiffness ratio. The conclusion of the PhD study is that flax fibres are an important source of cellulosic fibres. When the appropriate composite processing methods and the accurate test methods are used, flax fibre composites are demonstrated to be promising material candidates for structural applications as an attractive alternative to synthetic fibre composites.